The present invention is concerned with devices such as angular transducers or encoders for detecting the position of a rotating shaft relative to a stationary part such as a chassis, in particular for detecting the angular position of a rotor or shaft of an electric motor/generator relative to the stator of the motor/generator.
Important examples of high resolution encoders for motors are the EnDat series from the company Dr Johannes Heidenhain GmbH, 83301 Traunreut, Germany and SinCos systems from the company Max Stegmann GmbH, 78 156 Donaueshingen, Germany. Both permit a resolution of some million positions per turn by sending incremental analog position data in analog form while also providing the possibility to read absolute information in a serial digital format. The transmission of the analog signals used requires separate cabling between each encoder and the computer that controls the current sent to the motor. Such a cabling and the use of analog signals makes it simple to obtain a synchronisation between the time when the motor position is read and the time when the calculations for the next current controlling switch setting is performed. The analog position data are typically sampled in the same unit as calculates and sets the timing for the switches controlling the currents.
Presently used encoders for motor applications can be classified in two groups depending on if they have bearing system of their own or not. Encoders having an own bearing system can achieve very high resolutions at the expense of higher costs and a greater length in the axial direction of the motor. To avoid that the bearing system of the encoder creates large forces conflicting with the bearing system of the motor, the encoder chassis or rotor must be assembled against the motor using a flexible coupling. This normally makes the total system even longer and introduces possible resonances in the system.
Encoders without own bearings are limited to resolutions in the order of 8000 positions per turn against some millions for the high resolution devices. The company Renco and Danaher Controls, Gurnee, Ill., U.S.A., sells such encoders with the added benefit of integrated devices to adjust the position of the encoder disc relative to the encoder chassis during assembly. Such encoders normally transmit their information as digital incremental signals.
Angular transducers and encoders of a kind similar to those described herein are e.g. disclosed in the published International patent application WO 99/54683 and U.S. Pat. Nos. 5,998,783, 6,084,234 and 6,191,415 which are incorporated by reference herein.
An object of the invention is to provide a high resolution encoder with a low cost for cable assembly.
Another object of the invention is to provide a high resolution encoder that can be connected through a cable bus system while permitting a good synchronisation between the time when the motor position is read and the time when the calculations for the next current controlling switch setting are performed.
Yet another object of the invention is to provide a high resolution encoder that can be connected through a cable bus system.
Yet another object of the invention is to provide a high position resolution encoder having a high resolution also in measuring speed by ensuring a precise and known time interval between position sampling events.
Yet another object of the invention is to provide a high resolution encoder having a low production and assembly cost.
Yet another object of the invention is to provide a high resolution encoder having a high noise immunity inherent in digital information transfer.
Yet another object of the invention is to provide a high resolution encoder having a short total length.
Yet another object of the invention is to provide a short encoder having a multiturn registration ability.
The objects above are achieved by the invention, the characteristics of which appear from the appended claims.
Thus generally, an encoder for detecting the angular position of a shaft of an electric motor comprises an encoder disc having one or more encoding tracks having variations of some detectable physically detectable quantity along the periphery of the disc. The variations are sensed by one or more detectors in one or more reading heads. Analog position signals from the detectors are sampled in sampling-and-hold devices to give analog values, the sampling times given by sampling pulses issued by a sampling pulse device, e.g. a counter. The sampled analog values are converted to digital values, e.g. by first being time multiplexed and then passing an analog-to-digital converter, and then provided to a local processor, a microprocessor calculating position values. The local processor sends commands to the sampling pulse device to issue sampling pulses at accurately defined times, which are normally regularly spaced in time. The local processor also receives, from a central processor, read command signals commanding to it provide position values. These read command signals normally arrive rather regularly but are not synchronised to the sampling pulses. In order to provide positions values at suitable times after receiving the read command signals, the local processor is arranged to send its commands earlier or later, in relation to the normal regular rate.
This can be made by calculating the interval between the time of receiving a read command signal and the time when the last command for issuing a sampling pulse has been given, comparing the interval to a set value dependent on the processing times in the various components that the detected analog signals has to pass to give the calculated position values and taking the appropriate action when the calculated interval differs too much from the set value.
Furthermore generally, when controlling an electric motor having a shaft, the following steps are performed:
1. The angular position of the shaft is detected, e.g. optically or magnetically, such as generally by detecting physical variations in an encoding track, and based on the detected angular position an output analog signal is provided.
2. At commanded times, which are set to basically appear regularly, i.e. with a constant time interval between, starts the deriving, from the output signal, digital values of the angular position at those commanded times, and after finishing the deriving, the digital values are used in the controlling, e.g. are immediately sent to a central processor.
3. Request pulses, each for requesting a derived digital value, are issued, the request pulses generally appearing regularly in time but sometimes having deviations therefrom and not synchronised with the commanded times and not with the constant time interval between the commanded times.
4. The time interval between the commanded times is made to deviate from the constant time interval when the time period from the time of starting of deriving a value to a time when next receiving a request pulse is not within a predetermined range. After the deviation having been made, the constant time interval is again used for setting the next commanded time.
Obviously then, in the case where the time period from the time of starting to derive a value to the time when receiving a next request pulse is shorter than a lower limit of the predetermined range, a next commanded time is made to occur later. In the corresponding way, in the case where the time period from the time of deriving a value to a time when receiving a next request pulse is longer than an upper limit of the predetermined range, a next commanded time is made to occur earlier.
In the case where at least two electrical motors are controlled, values of the angular position of the shaft of each motor are derived and then, the derived values are sent in a time multiplexed way on a single bus cable for controlling the motors.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realised and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.